1. Field of the Invention
The present invention relates generally to solid-state microelectronics, and more particularly to pyroelectric radiation sensitive devices. The invention also relates to pyroelectric infrared light sensor structures adaptable for use with sensor packages or modules for detecting the presence of a human body in several applications including, but not limited to, alarm systems, intelligent air conditioners, autoadjustable illumination apparatus, and auto power-on audio systems.
2. Description of the Related Art
One conventionally known pyroelectric infrared sensor device is designated by the numeral 1 in FIG. 11. This infrared sensor device 1 includes electrodes 2a, 3a which are formed on the top and bottom surfaces of a rectangular-shaped pyroelectric body 1a with polarization being vertically effected between the electrodes 2a, 3a along the thickness as indicated by arrows shown. The electrodes 2a, 3a are disposed in the direction at right angles to the polarization direction.
In this conventional pyroelectric infrared sensor device 1, when rays of infrared light 9 are radiated from an object to be detected and are then incident on the light-receiving surface of the sensor device 1, slight temperature changes will occur in the pyroelectric body 1a. As a result, a packet of electrical charge carriers that has been accumulated on the surface of pyroelectric body 1a in the equilibrium state--i.e, A surface charge--might move, causing a voltage to arise accordingly. This voltage is usable in detecting the object to be detected by electrically amplifying it with impedance converter circuitry employing field effect transistors (FETs), by way of example, and then converting an amplified signal into a corresponding electrical signal.
Another conventional pyroelectric infrared sensor device is shown in FIGS. 12A to 12C, wherein this device is also designated by numeral 1. FIG. 12A is a plan view of the sensor device 1, FIG. 12B shows its side view, and FIG. 12C is the bottom view thereof.
As shown in FIG. 12A, the conventional pyroelectric infrared sensor 1 includes a substrate 2 made of a pyroelectric material. The pyroelectric substrate 2 has a top surface on which a pair of light-receiving electrodes 3a, 3b are disposed along with an interconnection electrode 4 connecting between them. As best shown in FIG. 12C, pyroelectric substrate 2 has a bottom surface on which a pair of spaced-apart electrodes 5a, 5b are formed overlapping the light-receiving electrode pair 3a, 3b on the top substrate surface. In the pyroelectric infrared sensor 1, polarization has been effected along the thickness direction of pyroelectric substrate 2.
One typical pyroelectric infrared sensor module 101 employing the pyroelectric infrared sensor device of FIG. 12 is shown in FIG. 13. This conventional pyroelectric infrared sensor module 101 includes a holder plate 102 which is manufactured by two-color formation techniques or electrode-on-ceramics formation techniques. The holder plate 102 has a surface with complicated configuration, on which a resistor chip 103 and an FET chip 104 as well as a pyroelectric infrared sensor device 105 are mounted. The holder plate 102 with resistor 103, FET 104 and sensor 105 mounted thereon is then attached to a support base 106. This base 106 is assembled for packaging with a cap 107 having on its top surface a light entrance window. The assembled sensor module 101 thus contains therein the electronics components 103 to 105 in an environmentally sealed manner.
Another conventional pyroelectric infrared sensor module 201 is shown in FIGS. 14A to 14C. This pyroelectric infrared sensor module makes use of the pyroelectric infrared sensor device of FIG. 12. Note here that FIG. 14A shows a plan view of the sensor module, FIG. 14B is its side view, and FIG. 14C is a bottom view of a holder plate 202 used in the module.
As shown in FIGS. 14A to 14B, the conventional pyroelectric infrared sensor module 201 comes with a pyroelectric infrared sensor device 205 which is attached on the upper surface of the holder plate 202. This holder plate has a lower surface on which a hightemperature resistance element 203 is mounted by conductive adhesion. The resistance element 203 is of the high-temperature baked type using carbon, thermet, a resistive material made of ruthenium oxide, or the like. An FET chip 204 is also mounted on the lower surface of holder 202 as shown in FIG. 14C. This holder plate 202 is attached to a support base 206 so that holder plate 202 is stably positioned over base 206 as best shown in FIG. 14B. A cap with a light entrance window (not shown) is assembled thereto for packaging.
In the conventional sensor modules 101, 201 employing the pyroelectric infrared sensor device 1, it is strictly required that the pyroelectric device per se be minimized in thickness while at the same time maximizing thermal insulation characteristics in order to improve the relative detection rate indicative of the characteristics of pyroelectric device. To maximize thermal insulation characteristics, it has been required that the light entrance section to which incoming rays of infrared light are introduced be formed into a hollow shape as shown in FIGS. 13A to 13C and 14A to 14C. To this end, it has been necessary to reduce or minimize the thermal capacity and thermal time constant of the prior art sensor modules 101, 201 for achievement of enhanced relative detection ratio.
In other words, with the conventional pyroelectric infrared sensor modules 101, 201, since polarization is principally carried out along the thickness direction of the sensor substrate, an attempt has been made to minimize the thermal capacity by forcing the device per se to decrease in thickness. This would result in a decrease in mechanical or physical strength while simultaneously reducing durability against thermal shocks applied thereto.
As far as performance is concerned, the pyroelectric body can directly receive external vibrations, mechanical shocks, and thermal shocks due to strong infrared light illumination, causing the pyroelectric infrared sensor device per se to receive mechanical vibrations, which in turn results in occurrence of noise. This might reduce the signal-to-noise (S/N) ratio degrading or lowering performance.
In short, since the thickness of pyroelectric infrared sensor device is reduced, rendering the resultant structure mechanically weakened, the mechanical and thermal reliability is undesirably degraded thus making the handling of pyroelectric infrared sensor devices very difficult.
In addition, the reduction in thickness of pyroelectric devices can increase the risk of occurrence of malfunctions due to the piezoelectric effect of the pyroelectric body per se. This might lead to a reduction in reliability. Therefore, it is difficult for the prior art pyroelectric infrared sensor modules 101, 201 to attain excellent performance with the S/N ratio enhanced.
With regard to fabrication, the mechanical or physical weakness of the pyroelectric body per se renders difficult both rigid support of the pyroelectric device and its retention in an accurate position during fabrication. More practically, it becomes difficult to accurately mount and support the pyroelectric sensor device by adhesion on an associated substrate at an intended location. This would also affect performance.
A further problem encountered with the conventional techniques is that specific thermal insulation (thermal resistivity) schemes are required for rendering the light-receiving section hollow in shape thus making it unable to easily form the pyroelectric light-receiving section. Moreover, because in most cases the thickness of the pyroelectric body per se is designed to fall within the range of 70 to 100 micrometers, for example, the resulting yield of production is lowered rendering extremely difficult the manufacture of such pyroelectric infrared sensor device itself.
A still further problem faced with the conventional techniques is an increase in manufacturing cost due to an increase in the number of parts or components required. More specifically, in the sensor modules 101, 201 of FIGS. 13A to 14C, necessary parts or components are increased in number due to the necessity of making use of the holder plate 102 or 202 with the high-temperature resistance element 203 formed thereon, for attachment of the resistor chip 103 thereto.
One conventional approach to avoidance of such problems is to employ a pyroelectric infrared sensor device 5 shown in FIG. 15. This sensor device 5 has an electrode arrangement forcing the polarization of pyroelectric body to be concentrated only at or near the infrared light incidence surface thereof. This structure is advantageous in view of the fact that the surface layers near the light-receiving surface contribute most of the output of the pyroelectric infrared sensor device.
More specifically, the conventional pyroelectric infrared sensor device 5 of FIG. 15 includes a pyroelectric body or substrate 6 of a rectangular shape, for example. Substrate 6 has a surface on which two electrodes 7, 8 are formed in such a manner that these oppose each other with a predetermined distance D being defined therebetween. Polarization treatment has been effected in advance between the two electrodes 7, 8 by application of a dielectric current (DC) voltage thereto. In this case, polarization tends to mostly occur at nearby portions of the substrate surface acting as the radiation receiving plane, causing the direction of polarization to be parallel with the infrared-light (9) incidence surface. In other words, the polarization direction is set identical to the "x" axis direction in FIG. 15. With such an arrangement, this conventional sensor device 5 is advantageous over those of FIGS. 11 to 12 in that the former enables polarization of pyroelectric body 6 to be concentrated only at or near the light-receiving surface of the pyroelectric substrate 6, which in turn leads to an improvement in relative detection rate.
Unfortunately, the above advantage of the conventional pyroelectric infrared sensor device 5 shown in FIG. 15 does not come without accompanying serious problems, which follow. While it is no longer necessary to make thinner the pyroelectric substrate per se, the area of its light-receiving section as well as th e electric capacitance (electrostatic capacitance) thereof remain less, rendering it difficult to obtain a sufficient pyroelectric current while also placing certain limitations on the possible improvement in sensitivity and in relative detection rate. Another problem in the conventional techniques is that the electric capacitance (electrostatic capacitance) remains smaller so that high-frequency noise components such as white noise are increased undesirably.
On the other hand, pyroelectric infrared sensor devices employing comb-shaped electrodes placed on pyroelectric substrates have been proposed. This kind of pyroelectric infrared sensor is disclosed in Japanese laid-open publication No. 7-198478. It has advantages, but is not able to avoid the foregoing disadvantages of conventional pyroelectric infrared sensor devices.